In recent years the heating efficiency of fuel-fired appliances such as household furnaces has been increased by extracting additional heat from the combustion gases before passing those gases into a flue external of the appliance. In the case of a gas-fired, forced-air furnace, for example, the combustion gases after passing through the usual heat exchanger cell, which also serves as part of a combustion chamber, are passed through one or more additional but non-fired heat exchanger cells and then into an external flue. Room air is forced past the exterior of all the exchangers so that heat exchange takes place through the metal walls of the exchangers. This heat exchange will typically reduce the temperature of the flue gases in the non-fired heat exchanger or recuperative cell to a level, for example 200.degree. F., at which some of the water vapor in the flue gases will condense on the inner surface of the recuperative cell. A drain connection is provided in the recuperative cell to allow the condensate to pass to a permanent drain. A recuperative furnace of this type is disclosed in U.S. Pat. No. 4,261,326.